Coating process

Abstract: The object of the present invention is to provide an electrostatic coating method employing a powder coating which provides for good transfer efficiency with consequent reduction of powder recovery load and, hence, is useful for conservation of resources and management of wastes. The present invention relates to a coating process applying a powder coating on the surface of the substrates formed with plastic or coated coat by electrostatic coating method wherein the temperature of said substrates is brought to a level not below the glass transition temperature of the component resin of said powder coating or the glass transition temperature of the component resin of of said plastic or coated coat prior to application of said powder coating.

Description: The present invention relates to a method of manufacturing a coated article according to the preamble clause of claim 1. Said coated article may include surface-coated metal or surface-coated plastics products such as for example automotive parts, building materials, constructional members, household appliance parts, and similar coated articles. A method of said type is known from document EP-A-0 372 740. In detail, said document discloses a method of coating a plastic substrate comprising the steps of: a) preheating the substrate at a temperature and for a time sufficient to degas the substrate; the temperature being above the temperature sufficient to cure a subsequently applied powder coating composition; b) applying immediately a powder coating composition to the preheated substrate above the temperature sufficient to cure the powder coating composition; and c) heating immediately the coated substrate at a temperature and for a time sufficient to cure the powder coating composition. The times and temperatures for this known preheating step will vary somewhat depending on the identity of the substrate and the powder coating composition. Typically, the temperature will be at least 149° C, and preferably 163 to 204° C, and the preheating time typically will be at least 15 minutes, usually from 15 to 60 minutes. After the preheating step, the powder coating composition is immediately (i.e., before the substrate cools below the curing temperature of the powder coating composition) applied to the preheated substrate. Preferably, the powder coating composition may be applied by electrostatic spraying. After the powder coating composition has been applied, the coated substrate is then heated immediately (i.e., before the substrate cools below the curing temperature of the powder coating composition) at a temperature and for a time sufficient to cure the powder coating composition. At no time during the coating process should the temperature of the substrate be allowed to drop below the curing temperature of the powder coating composition. When a plastics substrate is coated with a powder coating composition by means of a powder coating process, there is a danger of the so-called "popping", which may occur due to a surface porosity of the substrate. "Popping" results in a removal of small or larger pieces of the coating layer and deteriorates the appearance of the coating surface. The fore-mentioned coating method (which is also disclosed in Japanese Kokai Publication Hei-2-194878) seeks to avoid said popping by heating the substrate to a temperature sufficient for degassing prior to the application of the coating layer(s). However, the proposed heating temperature of the plastics substrate is too high, because the powder coating composition will inevitably tend to undergo partial curing, which may deteriorate the appearance of products. Japanese Kokoku Publication Sho-51-43152 discloses a method for applying a top coating layer by means of a powder coating step subsequently to the formation of a base coating layer comprising a thermosetting resin coating layer. Said method includes a step of heating the substrate to reduce the content of volatile components of the base coating layer to an amount not more than 6 % by weight to thereby improve the metallic tone and durability of the coating layer(s) as well as the transfer efficiency of the powder coating process. The above mentioned methods intend to improve the coating processes employing powder coating steps, but, while the content of the volatile component of the base coating layer is limited to, say, 6 % by weight or less on the one hand, an improvement of the transfer efficiency is limited by the remaining content of volatile matters. On the other hand, an excessive heating of the substrate in order to decrease the content of volatile matter may impair the powder coating process and may lead to a deterioration of the appearance of the coated articles. Therefore, the transfer efficiency is not actually improved in an extent as desired. The above-mentioned problem of improving the transfer efficiency remains virtually unsolved. Therefore, the technical problem (object) of the present invention is to improve the transfer efficiency of the powder coating composition in a method of manufacturing a coated article of the above-stated generic kind, without an undue deterioration of the appearance of the coated article. Starting from a method of manufacturing a coated article, comprising the steps of providing an article comprising a substrate made of plastics material or comprising a substrate made of plastics or metal and having a base coating layer containing a plastics component, preheating said substrate to a given temperature, applying a powder coating material comprising a resin component by means of an electrostatic coating process to said preheated substrate in order to obtain a powder coating layer, a solution of the fore-mentioned technical problem according to the present invention is characterized in that in order to increase a transfer efficiency of the powder coating material in the electrostatic coating process, the substrate is preheated to a temperature of from 40° C to 140° C, but within said temperature range to a temperature higher than the glass transition temperature of said plastics material forming said substrate, or higher than the glass transition temperature of said plastics component of said base coating layer; higher than the glass transition temperature of said resin component of said powder coating material; and lower than the curing temperature of said resin component of said powder coating material; lower than the curing temperature of the plastics component of the base coating layer when a substrate made of plastics or metal having a base coating layer is used. According to a preferred embodiment of the method according to the present invention, the article may comprise a substrate made of plastics or metal, and said substrate comprises a base coating layer containing less than 10 % by weight volatile components. Further, said base coating layer may be obtained by an electrodeposition coating process or by an aqueous coating process, and it is preferred that said base coating layer containing less than 2 % by weight of vola-tile amine compound based on the total weight of volatile matter. The method according to the present invention is now described in more detail. The coating process according to the present invention may be applied to the surface of plastics substrates. Moreover, the coating process according to the present invention may be applied to the surface of a metal or of plastics substrate subsequently to the deposition of a coating layer forming a base coating layer on said substrate surface. The forming of said base coating layer may be effected by known processes, such as electrodeposition coating process, applying an aqueous coating mixture, solvent type coating process or powder coating process. In case of a substrate made of metal, said metal may be selected for example from a group comprising iron materials and other electrically conductive metallic materials; an especially preferred metallic stubstrate is an iron phosphate-treated steel sheet or a zinc phosphate-treated steel sheet. The surface of a plastics substrate and/or the surface of a base coating layer applied on the surface of said substrate made of metal or plastics forms the surface of the substrate for the application of the coating process according to the present invention. Therefore, and within the present specification said surfaces are collectively referred to as "the surface of the substrate made of plastics or the surface of a base coating layer". There is no particular limitation with respect to the powder coating materials which may be used with the method of manufacturing a coated article according to the present invention. Typically, the powder coating material comprises a base material or vehicle and pigments. The vehicle, for example, may be selected from a group comprising polyester resins, acrylic resins, epoxy resins and other resins. In said vehicle, pigments and additives are incorporated in such a manner as to provide a powder comprising a content of 100 % of non-volatile components. The particle size of the powder forming the powder coating material, in terms of a bulk average particle diameter may range of from 5 µm to 50 µm and may preferably range of from 8 µm to 40 µm. In case where the average particle diameter ranges of from 5 µm to 20 µm, the amount of particles having a particle diameter less than 5 µm is preferably less than 25 % by weight. In case where the average particle diameter ranges of from 20 µm to 50 µm, the standard deviation of the particle size distribution is preferably not larger than 20 µm. The standard deviation of a particle size distribution is expressed by the following term: [Σ {(D - X) 2 F} / Σ F] 1/2 , wherein: D means the diameter of each individual particle, X means the bulk average particle diameter; X means Σ (D F) / Σ F; and F means the occurence of particles. Further, the type of plastics material forming the substrate is not critical. Thus, the substrate may comprise thermoplastic or a thermosetting plastics material; said plastics material may be reinforced, but there is no need of reinforcement. This means, the plastics material forming the substrate may be selected, for example, from a group of plastics, including phenolic resins inclusive phenol-cellulose versions, silicone resins, amino resins, polyurethanes, polystyrenes, polypropylenes, thermoplastic acrylic resins, polyvinyl chlorides, polyacrylonitriles, polybutadienes and acrylonitrile-butadiene copolymers. In case where the plastics material forming the substrate comprises reinforcement fibers, said fibers may comprise boron fibers and other fibers, however with the exception of glass fibers. In the method of manufacturing a coated article according to the present invention, the temperature of the substrate is increased to a temperature of from 40° C to 140° C, but within said temperature range to a temperature higher than the glass transition temperature of said plastics material forming said substrate, or higher than the glass transition temperature of said plastics component of said base coating layer; higher than the glass transition temperature of said resin component of said powder coating material; and lower than the curing temperature of said resin component of said powder coating material; lower than the curing temperature of the plastics component of the base coating layer when a substrate made of plastics or metal having a base coating layer is used. When the substrate temperature is below 40° C, the transfer efficiency is poor; a substrate temperature higher than 140° C may cause an advanced partial curing of the powder particles forming the powder coating material, which may deteriorate the appearance of the finished coated product. In order to increase the temperature of the substrate to such a level, the substrate can be preheated prior to applying the powder coating material. When providing a base coating process including a baking step, the subsequent powder coating step may be performed before a renewed cooling step following the baking step in course of a substrate base coating process. Generally speaking, the above-mentioned temperature of the heated substrate is preferably not higher than the upper limit of the baking temperature of the powder coating process. In case where the substrate surface comprises a base coating layer, the upper limit of said temperature is preferably not higher than the upper limit of a baking temperature of a process for preparing said base coating layer and not higher than the upper limit of the baking temperature for the powder coating process. In case where the powder coating layer is to be deposited directly on the surface of a substrate made of a plastics material, the upper limit of the fore-mentioned temperature is preferably lower than the deformation temperature of said plastics material. In a preferred embodiment of the present invention, where the surface of the substrate comprises a base coating layer, the content of volatile components of said base coating layer is controlled such that said base coating layer does not contain more than 10 % by weight volatile components based on the weight of said base coating layer. If this limit of 10 % by weight volatile components is exceeded, foaming, surface roughness and yellowing may occur, which may deteriorate the appearance of the final product. The preferred limit of volatile components is 5 % by weight or less. In another preferred embodiment of the present invention, where the surface of the substrate comprises a base coating layer, which has been formed by an electrodeposition process or by applying an aqueous coating mixture, said base coating layer may contain a volatile amine compound. The content of said volatile amine compound is controlled such that said base coating layer does not contain more than 2.0 % by weight of said volatile amine compound based on the weight of the non-volatile matters contained within said base coating layer. If this limit of 2.0 % by weight volatile amine compound is exceeded, the appearance of the final product will not be as good as desired. The preferred limit of said volatile amine compound is 1.5 % by weight, and still more preferred is an upper limit of 1.2 % by weight. In accordance with the present invention, where the surface of the substrate being made of a plastics material and/or comprising a base coating layer containing at least one resin component, said surface is first heated to the above-mentioned temperature, and then an electrostatic coating process is performed in order to deposit a powder coating layer. The electrostatic coating process as such can be carried out in a conventional manner. In the practice of the present invention, an electrically conductive particulate substance, such as graphite powder, may be incorporated in the plastics material forming the substrate in order to impart electric conductivity. A similar effect may be achieved by using an electrically conductive reinforcing fiber as a component of the substrate material. If necessary, the plastics substrate may be coated with an electrically conductive primer or with an electically conductive wash solution to impart electric conductivity. EXAMPLES The following examples are intended to describe the present invention in further detail and shall by no means be construed as defining the scope of the invention. Reference Example 1: Preparation of an aqueous metallic coating mixture A composition is prepared by adding 30 parts by weight of Cymel 303 (methoxylated methylolmelamine, Mitsui Toatsu Chemicals, Inc.) to 140 parts by weight of an aqueous acrylic resin having a hydroxyl value of 20 and an acid value of 58 (amine-neutralized, non-volatile matter 50%). Thereafter, 15 parts by weight of an aluminum pigment paste (AW-666, Asahi Chemical Industry Co., Ltd.) are added to this composition, in order to obtain a mixture. The mixture is stirred in order to obtain an aqueous metallic coating mixture. Reference Example 2: Preparation of an acrylic powder coating material A composition is prepared by adding 80.5 parts by weight of decane-dicarboxylic acid, 4 parts by weight of a surface conditioner and 2 parts by weight of benzoin to 315 parts by weight of a glycidyl group-containing acrylic resin (glass transition temperature (Tg): 52° C). Following a melt-compounding step, the composition is finely divided in order to provide an acrylic powder coating material. Reference Example 3: Preparation of a polyester powder coating material A composition is prepared by adding 7.5 parts by weight of triglycidyl isocyanurate, 60 parts by weight of titanium dioxide, 0.4 parts by weight of a surface conditioner and 1.1 parts by weight of benzoin to 100 parts by weight of a carboxyl group-containing polyester resin (Tg: 63° C). Following a melt-compounding step, the composition is finely divided in order to provide a polyester powder coating material. Example 1: An iron phosphate-treated steel sheet is electro-coated (Powertop, U Series, Nippon Paint Co., Ltd.) providing a base coating layer having a coating thickness of 20 µm. In a further coating step (OTOH Series, Nippon Paint Co., Ltd.) an intermediate coating layer is deposited having a coating thickness of 35 µm. The steel sheet comprising the base coaring layer and the intermediate coating layer is then coated with the aqueous metallic coating mixture as prepared in Reference Example 1 and is baked at 80° C for 10 min., at 100° C for 10 min. and at 120° C for 10 min. Thereafter, samples of this coated and baked sheet are brought to the different temperatures as stated in Table 1. Thereafter, an electrostatical coating step is performed using the acrylic powder coating material as prepared in Reference Example 2 (coating conditions: applied voltage -80 kV; delivery rate 120 g/min). Thereafter, a transfer efficiency is evaluated for each sample. The results are stated in Table 1. Substrate temperature: 25° C 60° C 100° C 120° C Transfer efficiency: 51 % 63 % 78 % 81 % The above results indicate that an improvement in transfer efficiency may be realized, when the temperature of the substrate is maintained at a level not lower than the glass transition temperature (Tg) of the acrylic resin contained in the powder coating material. Example 2: A plastics blank intended for the use of an automotive bumper and made of a plastics material having a glass transition temperature (Tg) of 90°C (Mitsui Petrochemical Co., Ltd.) is treated with trichloroethane and coated with a conductive primer (RB-1140 CD primer, Nippon Paint Chemical Co., Ltd.). This plastics blank is brought to the different temperatures as shown in Table 2. Thereafter, an electrostatical coating step is performed using the acrylic powder coating material as prepared in Reference Example 2. The coating conditions are the same as stated in Example 1. Thereafter, the transfer efficiency is evaluated for each sample. The results are shown in Table 2. Substrate temperature: 25 ° C 60° C 100° C Transfer efficiency: 48 % 60 % 82 % The above results indicate that an improved transfer efficiency may be obtained when maintaining the substrate temperature at a level not lower than the glass transition temperature (Tg) of the acrylic resin component of the powder coating material. An even more remarkable improvement of the transfer efficiency may be obtained when maintaining the substrate temperature at a level not lower than the glass transition temperature (Tg) of the plastics material forming the substrate. Example 3: As in Example 1, an iron phosphate-treated steel sheet is electrocoated, then intermediate-coated and further coated with the aqueous metallic coating mixture as prepared in Reference Example 1. Thereafter, samples of the coated sheet are preheated at 80° C for 5 min., at 80° C for 10 min., at 90° C for 5 min., at 100° C for 5 min., at 130° C for 5 min., and at 140° C for 5 min. Then, without performing a cooling step, the sheet samples are coated with the acrylic powder coating material as prepared in Reference Example 2. The coating conditions are the same as stated in Example 1. Thereafter, each sample is baked at 150° C for 25 min. Thereafter, the content of the non-volatile components (% by weight) and the content of the volatile amine compound (% by weight based on the total content of non-volatile matters) of the intermediate coating layer obtained from the aqueous metallic coat are determined, following the pre-heating step. In addition, the appearance of the final product is rated. The appearance is indicated by the value of NSIC measured by portable measuring instrument of sharpness of the reflection (Sugai Shikenki Co.). The results are stated in Table 3. Preheating conditions 80°C 5 min. 80°C 10 min. 90°C 5 min. 100°C 5 min. 130°C 5 min. 140°C 5 min. Non-volatile matter (% by weight) 85 90 88 90 95 99 Volatile amine compound (% by weight based on the content of total nonvolatile matter) 2.5 2.1 1.5 1.2 0.6 0.3 Appearance (NSIC) 58 70 71 73 74 75 It is apparent from Table 3, that a final product comprising a good appearance may be obtained by controlling the content of volatile components of the intermediate coating layer to be not larger than 10 % by weight, and/or controlling the amine content of said intermediate coating layer to be not larger than 2 % by weight based on the weight of the total content of non-volatile matters forming said intermediate layer. Example 4: A zinc phosphate-treated steel sheet having a thickness of 0.6 mm is coated with the same intermediate coating layer as used in Example 1, providing an intermediate coating layer having a coating thickness of 35 µm. Samples of this coated sheet are brought to the different temperatures as indicated in Table 4. An electrostatical coating step is performed using the polyester powder coating material as prepared in Reference Example 3. The coating conditions are the same as stated in Example 1. Thereafter, the transfer efficiency is evaluated for each sample. The results are stated in Table 4. Substrate temperature: 20°C 70°C 120°C Transfer efficiency: 58 % 70 % 81 % With the method of manufacturing a coated article according to the present invention, the transfer efficiency of a powder coating process is enhanced and the coating powder recovery is improved by preheating the substrate to a temperature not below the glass transition temperature (Tg) of the resin component of the powder coating material. Therefore, the invention is of value for the prevention of pollution and management of wastes. Furthermore, in the sequence of production steps of providing a steel sheet, applying primer, baking step, cooling step, applying a powder coating layer, the cooling step may be omitted. In the sequence of treatment steps for coating automotive parts, comprising applying an aqueous base coating layer, preheating step, cooling step, applying a powder top clear coating layer, the cooling step may be omitted in a similar manner. Therefore, the present invention provides advantages inter alia in saving labor and energy.

Claims: A method of manufacturing a coated article, comprising the steps of providing an article comprising a substrate made of plastics material or comprising a substrate made of plastics or metal and having a base coating layer containing a plastics component, preheating said substrate to a given temperature, applying a powder coating material comprising a resin component by means of an electrostatic coating process to said preheated substrate in order to obtain a powder coating layer, characterized in that in order to increase a transfer efficiency of the powder coating material in the electrostatic coating process, the substrate is preheated to a temperature of from 40° C to 140° C, but within said temperature range to a temperature higher than the glass transition temperature of said plastics material forming said substrate, or higher than the glass transition temperature of said plastics component of said base coating layer; higher than the glass transition temperature of said resin component of said powder coating material; and lower than the curing temperature of said resin component of said powder coating material; lower than the curing temperature of the plastics component of the base coating layer when a substrate made of plastics or metal having a base coating layer is used. The method according to claim 1, wherein said substrate comprising a base coating layer; and said base coating layer containing less than 10 % by weight volatile components. The method according to claim 1 or 2, wherein said substrate comprising a base coating layer obtained by an electrodeposition coating process or by an aqueous coating process; and said base coating layer containing less than 2 % by weight of a volatile amine compound based on the total weight of non volatile matter.

Inventors: INOUE KOICHI, LIBKE SHANNON, TSUTSUI KOICHI, INOUE, KOICHI, LIBKE, SHANNON, TSUTSUI, KOICHI
Applicants: NIPPON PAINT CO LTD, NIPPON PAINT CO., LTD.
Application year: 1994
Main classification: B05D1
IPC codes: B05D1, B05D7